Existing windshield anti-ice systems and fog control systems typically include a thin resistive layer made of a semiconductor material deposited between transparent inner and outer windshield surfaces. Typically, the inner windshield surface is made of polycarbonate and the outer windshield surface may be made of glass, polycarbonate, herculite acrylic or other suitable material. The inner windshield surface is formed into the desired final shape, and the resistive layer is deposited on the external facing side of the inner windshield surface. For example, the resistive layer may be made of indium tin oxide which is sprayed on the inner windshield surface. Thereafter, an adhesive interlayer of a known film adhesive, such as urethane, is deposited on the resistive layer to secure the outer windshield surface.
Referring to FIG. 1, existing anti-ice systems typically feature a single channel, isolated, analog controller 10 that pulses electric current to a windshield heating film 20 based on the temperature of windshield interior surface. One or two temperature sensors 12 are provided to monitor the windshield interior surface temperature, and when the temperature falls below a threshold level, the controller 10 activates a relay coil 17 which in turn activates a normally open contact 18 of a relay 19 to apply current to the resistive layer 20, thereby heating the windshield 22. When the windshield interior temperature exceeds a threshold value, the controller switches off the relay 19 to remove the current from the resistive layer 20.
There are several draw backs to the existing method of deicing and defogging a windshield. First, the controller and relay are heavy, exacting a weight penalty which is particularly important on aircraft. Additionally, the temperature threshold for turning off the controller is based on the worst case conditions which the windshield will encounter, e.g. the coldest, highest moisture content, fastest airspeed, etc. This results in a windshield temperature that is almost always higher than necessary to adequately perform the deice and/or defog function. Additionally, unnecessarily high windshield temperatures adversely affect the IR signature of the windshield, which is particularly important in the case of military vehicles.
Another problem associated with existing methods of deicing and defogging a windshield is that isolated analog controllers have little available fault tolerance. Therefore, erroneous sensor data or an errant controller function may lead to an over temperature condition which may be destructive to the windshield optical properties. In order to prevent over temperature of the windshield from occurring, the vehicle operator, e.g., pilot, must pull a system circuit breaker or take some other affirmative action to prevent the over temperature from occurring. Over temperature conditions can affect the heating film in such a manner to seriously tint the transparency of the windshield. Additionally, in windshields containing an acrylic outer layer, an over temperature condition can potentially de-laminate and shrink the outer transparent ply.